You can perform OS reinstallation at anytime in client portal and here are the steps:
1) Login to client portal
2) Select "Services > My Services" after login to client portal
3) Click on the relevant server from the services list in "My Products & Services" page.
4) Sroll down the "Manage Product" page and look for "Reinstall" button.
5) Select the OS that you wanted to reinstall and press the reinstall button.
6) Enter root password and click "Confirm".
** Please take note that all data in current server will be permanently wipe off. Make sure you backup your data before proceed.
7) Wait for 3-5 minutes before the server back online. You can always ping to your IP address to confirm.
Please open support ticket if you experience any issue.